The present invention generally relates to methods for processing metal alloys. More particularly, this invention relates to a method for producing forged superalloy articles, in which fine grain sizes in the forged article can be retained following a supersolvus heat treatment, such that the articles are characterized by a microstructure with a desirable grain size.
Rotor components of land-based gas turbine engines used in the power generation industry are often formed of iron-based or nickel-based alloys. For certain advanced land-based gas turbine engines, such as the H and FB class gas turbines of the General Electric Company, rotor components are currently formed from gamma double-prime (γ″) precipitation-strengthened nickel-based superalloys, such as Alloy 718 and Alloy 706. For example, wheels (disks) and spacers have been formed from cast ingots that are billetized and forged either above or below the solvus temperature of the alloy (typically in a range of about 1750 to about 2100° F. (about 954 to about 1150° C.)) to obtain the desired outline for the component. The best current processing practices typically result in relatively coarse-grained microstructures in the billet (for example, ASTM 00 or larger) as well as the finished forging (for example, ASTM 2 to 7) (reference throughout to ASTM grain sizes is in accordance with the standard scale established by the American Society for Testing and Materials). While coarse grains are desirable for certain regions/components, finer grains are often desirable for other regions/components. For example, while disks benefit from having relatively coarse grains at their rims to promote their resistance to creep and fatigue crack growth, their hubs (bores) benefit from finer grains to promote their resistance to low cycle fatigue (LCF) and burst properties.
Rotor components for aircraft gas turbine engines have often been formed by powder metallurgy (PM) processes, which are known to provide a good balance of creep, tensile and fatigue crack growth properties to meet the performance requirements of aircraft gas turbine engines. Typically, a powder metal component is produced by consolidating metal powders in some form, such as extrusion consolidation or hot isostatic pressing (HIP), to yield a fine-grained billet (for example, ASTM 8 or finer). The billet is then isothermally or hot die forged at a temperature slightly below the gamma-prime solvus temperature of the alloy to approach superplastic forming conditions, which allows the filling of the die cavity through the accumulation of high geometric strains without the accumulation of significant metallurgical strains. The forging process generally retains the fine grain size within the material while obtaining the desired outline for the component, after which a final heat treatment is performed before finish machining to complete the manufacturing process. Unlike advanced turbine systems for land-based gas turbine engines, PM rotor components for aircraft gas turbine engines have been typically formed from gamma prime (γ′) precipitation-strengthened nickel-based superalloys with very high temperature and stress capabilities demanded by those parts. In order to improve creep resistance, fatigue crack growth resistance and other mechanical properties at elevated temperatures, the final heat treatment of these alloys may be performed above their gamma prime solvus temperature (generally referred to as supersolvus heat treatment) to cause significant coarsening of the grains. The desirability of achieving relatively coarse grains at the rim of a rotor disk and relatively finer grains within its hub is evidenced by U.S. Pat. No. 5,527,020 to Ganesh et al., which discloses a heat treatment process and apparatus for selectively heat treating the rim of a disk in order to cause grain growth in the disk while maintaining a finer grain structure in the hub.
The nickel-based superalloy rotors used in large electrical power generating turbines have generally not required the higher temperature gamma prime alloys nor this grain coarsening process to meet their mission and component mechanical property requirements, though it is foreseeable that such higher temperature alloys could be required at some future date to increase turbine efficiencies or increase component life.
During conventional manufacturing procedures involving hot forging operations, a wide range of local strains and strain rates may be introduced into the material that can cause non-uniform critical grain growth during post forging supersolvus heat treatment. Critical grain growth (CGG) as used herein refers to random localized excessive grain growth in an alloy that results in the formation of grains whose diameters exceed a desired grain size range for an article formed from the alloy. The presence of grains that significantly exceed a desired grain size range can significantly reduce the low cycle fatigue resistance of the article and can have a negative impact on other mechanical properties of the article, such as tensile and fatigue strength. U.S. Pat. No. 4,957,567 to Krueger et al. teaches a process for eliminating critical grain growth in fine grain nickel-based superalloy components by controlling the localized strain rates experienced during the hot forging operations. Krueger et al. teach that local strain rates must generally remain below a critical value to avoid critical grain growth. Further improvements in the control of final grain size have been achieved with the teachings of U.S. Pat. No. 5,529,643 to Yoon et al., which places an upper limit on the maximum strain rate gradient during forging, and U.S. Pat. No. 5,584,947 to Raymond et al., which teaches the importance of a maximum strain rate and chemistry control. For example, Raymond et al. teach an upper limit strain rate of below about 0.032 per second (s−1) for the gamma prime nickel-based superalloy commercially known as René 88DT (U.S. Pat. No. 4,957,567). U.S. Published Patent Application Ser. No. 2009/0000706 to Huron et al. teaches that, by increasing the carbon content of Renè 88DT, strain rates of up to about 0.1 s−1 are possible without critical grain growth.
The above efforts evidence the importance of grain size control in alloys used to form rotating components that are used in high temperature and high stress applications where resistance to creep and fatigue are critical. Controlling grain size along with the other mechanical properties of the alloys can have a direct effect on component life and cost. However, an ongoing challenge with components such as disks is the simultaneously desire to promote creep life with coarser grains and promote fatigue life with finer grains. As noted above, typical forgings produced from cast and wrought ingots typically have a relatively coarse final grain size (for example, ASTM 2 to 7), which are difficult to refine to form smaller grain sizes that are desired for the hub region of a rotor disk. On the other hand, forgings produced from PM billets yield much finer grain sizes (for example, ASTM 8 or finer), which then require a heat treatment capable of coarsening the grains in the rim without resulting in critical grain growth. Accordingly, significant challenges are encountered when attempting to achieve different grain sizes within a single forging.